Polyvinyl alcohol (hereinafter abbreviated as PVA) resins have excellent gas barrier properties, toughness, and transparency. Therefore, they are preferably used for packaging materials for various articles.
However, since the melting point and the decomposition point of each of the PVA resins are close to each other, the PVA resins cannot be melt-molded substantially. Therefore, several limited methods can be employed for PVA resin moldings, which include: a method in which a water solution of PVA resin is poured on a substrate and dried, thus, obtaining a resultant film; and a method in which a water solution of PVA resin is coated on a substrate, and dried, resulting in a desired coated substrate. Such a limitation greatly prevents the development of PVA resins from applying the resin for the wide use as wrapping materials.
Recently, a PVA resin that can be melt-molded and has excellent gas barrier properties has been proposed. This PVA resin has an 1,2-diol unit in its side chains. See, for example, Patent document 1.
A PVA resin has a high melting temperature and has excellent gas barrier properties, and such characteristics would be resulted from the followings: the resin has a simple molecular structure, and thus has high crystallinity; and molecular chains of the resin are strongly confined by hydrogen bonds formed from hydroxyl groups contained in its crystalline portions and non-crystalline portions. The PVA resin described in Patent document 1 has lower crystallinity because of the steric hindrance caused by its side chains, thus has lower melting temperature. However, degradation of gas barrier properties resulting from the lower crystallinity is assumed to be suppressed by means of strong hydrogen bonds formed from side chain-hydroxyl groups contained in the non-crystalline portions.
However, the high crystallinity and the strong confinement of PVA resin molecules lead to poor flexibility and poor impact resistance when such a PVA resin is compared with other thermoplastic resins. The PVA resin described in Patent document 1 has been improved with respect to such problems to a certain degree by means of the introduction of 1,2-diol unit to the side chains. However, the PVA resin is still insufficient for practical use.
Ethylene-vinyl alcohol copolymer (hereinafter abbreviated as EVOH) has a structure in which ethylene units are introduced into the main chain of PVA resin molecule, and such a structure provides the resin with melt-moldability. Therefore, EVOH is widely used for various packaging materials although it has relatively poor gas barrier properties compared with PVA resins. It would be greatly desired to provide EVOH with flexibility and impact resistance similar to the case of PVA resins.
For such purposes, various studies have been widely conducted, in which a thermoplastic elastomer is blended with a PVA resin or EVOH, resulting in a formation of a sea-island structure, wherein the PVA resin or EVOH provides the sea part and the thermoplastic elastomer provides the island part, thus, improving the flexibility and impact resistance without impairing the gas barrier properties.
For example, a thermoplastic resin composition with improved flexibility and impact resistance is proposed, in which a hydrogenated block copolymer as a thermoplastic elastomer is blended with a water-soluble PVA rein, wherein the hydrogenated block copolymer is obtained by hydrogenation of a block copolymer that contains two or more of blocks of aromatic vinyl polymer and one or more of blocks of conjugated diene polymer. See, for example, Patent document 2.    [Patent document 1] JP2004-075866A    [Patent document 2] JP2001-114977A